The present invention generally relates to DC to DC converters and more particularly to a buck/boost pulse width modulated regulator which is operable to provide a desired output voltage over a wide range of input voltage including voltages greater than or less than the desired output voltage.
Regulated DC to DC converters utilizing pulse width modulation have taken several configurations in the prior art. The general requirements for such converters are for high efficiency, small converter size and weight, good output voltage regulation, small output ripple, and the capacity to withstand wide load variations. Additional requirements are often dictated by the specific application of the converter. Perhaps the next severe requirements imposed on such converters are those dictated by space applications as, for example, in a communications or meteorological satellite. In such applications, the input power supply voltage is supplied by batteries which are charged by solar cell arrays. Since the solar cell arrays are periodically shaded by the earth, the state of charge of the batteries and hence the input voltage varies widely. Because of the wide variation in input voltage, the DC to DC converter in this particular application must be capable of providing both buck and boost in order to provide a continuously regulated output voltage of the desirec value.
One previous approach was to provide either a buck followed by a boost or a boost followed by a buck. In either case, the boost circuit comprised a center tapped transformer winding the ends of which were connected to ground through controlled switches. The ends of the transformer winding were also connected by power diodes to an inductor and a capacitor which acted as storage elements. The voltage across the capacitor was supplied to a pulse width modulator to alternately open and close the switches connected to the ends of the transformer winding. Input voltage to the boost circuit was supplied to the center tap of the transformer winding. The buck circuit included another inductor supplied with an input voltage via a third controlled switch. This inductor supplied voltage to another capacitor, and the voltage across this capacitor was sensed by a second pulse width modulator which controlled the opening and closing of the third switch.
There are several disadvantages associated with this approach whether implemented as a buck followed by a boost or a boost followed by a buck. First of all, two pulse width modulators are required, one for the boost circuit and one for the buck circuit. This, of course, complicates the circuit with an attendant lowering of both reliability and efficiency. Second, separate inductors and capacitors are required in each of the buck and boost circuits which greatly adds to the bulk and weight of the converter. Third, considerable care must be exercised in the design of the boost circuit to prevent the simultaneous conduction (overlap) of the controlled switches connecting the ends of the transformer winding to ground. In addition to these disadvantages, high ripple currents in more than one capacitor are produced, and the maximum voltage of the switches connected to the ends of the transformer winding and the boost circuit is twice the input voltage.
Another approach was to provide an energy storage boost followed by a buck or vice versa. In either case, the energy storage boost circuit comprised an inductor connected in series with a power diode between positive input and output terminals. An energy storage capacitor was connected across the output terminals, and a controlled switch was connected between the junction of the inductor and the power diode and ground. A pulse width modulator was connected across the output of the capacitor and controlled the opening and closing of the switch. The buck circuit included a controlled switch connected in series with an inductor between positive input and output terminals with a capacitor connected across the output. A power diode was connected between the junction of the controlled switch and the inductor and ground. A second pulse width modulator was connected across the capacitor and controlled the opening and closing of the switch supplying the inductor of the buck circuit. Again, there are several disadvantages to this approach. First, two pulse width modulators are required. Second, additional inductors and capacitors are required. Third, high ripple currents are produced in more than one capacitor.
Another approach was to use a phase controlled buck/boost configuration. This arrangement has some similarities to the first described circuit except that the pulse width modulators are replaced by oscillators. A free running oscillator controls the switches which are connected to either end of the transformer winding in the boost circuit. A single inductor and capacitor are used as storage elements for both buck and boost, and the power diodes connected to either end of the transformer winding are connected by third and fourth controlled switches to the inductor. A phase controlled oscillator is connected across the output capacitor to control the opening and closing of the third and fourth switches.
While the phase controlled buck/boost approach does have the advantage of eliminating one of the inductors and capacitors, two oscillators are still required to control the several switches. Care must be exercised in design of this type of circuit to prevent the simultaneous conduction (overlap) of the switches connected to either end of te transformer winding. Moreover, these switches operate at twice the input voltage.
There has been yet another approach to providing both buck and boost in a DC to DC converter which results in considerable simplification over any of the circuits described thus far. This approach may be characterized as the energy storage approach and is typically implemented with a transformer having a primary winding connected in series with a controlled switch across a source of input voltage. The secondary winding of the transformer is connected by a power diode to charge a storage capacitor connected across the output of the converter. A pulse width modulator is connected across the capacitor and controls the switch in series with the primary winding to provide a regulated output voltage. While very appealing in its simplicity, this approach has some significant disadvantages including high ripple currents at the output of the converter and high peak currents through the controlled switch in series with the primary of the transformer. In addition, this switch operates at greater than the input voltage.
Not only is it desirable in a space application, such as a satellite power supply, to simplify the circuitry for reasons of bulk and weight limitations, but it is vitally important that both the efficiency and the reliability of the circuit be as high as possible. Obviously, increased efficiency leads to lower power source requirements and substantially less heat dissipation. Since the controlled switches of the DC to DC converters are typically switching transistors, lowering of the maximum voltages across these transistors and the maximum current through them will result in improved reliability of the power supply.